We’re all familiar with the 250 knot speed restriction under 10,000 feet. Specifically, according to FAR 91.117(a),
Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 mph).
As a rule of thumb, the lower the altitude, the more congested the airspace. This results in the need for slower aircraft. A few decades ago, multiple mid-air collisions occurred which pushed the FAA to define better speed restrictions at the lower altitudes that needed them.
Additionally, FAR 91.215 does not require Mode C transponders for some aircraft below 10,000 feet MSL. This limits the information that ATC has for each aircraft, another reason for the 250 knot speed restriction.
The 250 knot restriction is not the only one under 10,000 feet. As per FAR 91.117(c), no pilot flying under a Class B airspace or in a VFR corridor through a Class B airspace may fly at or over 200 knots. There are multiple overarching speed restrictions just like this one that are put in place during different circumstances.